The present invention relates to an electric discharge lamp, comprising an envelope at least part of which consists of a transparent wall whose inner surface is at least partly covered with a coating of luminescent material capable of emitting visible light due to the effect of being irradiated by ultraviolet excitation radiation, the lamp furthermore comprising means for producing such radiation.
Such an electric lamp, also commonly denoted by the term xe2x80x9cfluorescent lampxe2x80x9d or xe2x80x9cfluorescent tubexe2x80x9d, is usually made in tubular form, its envelope generally consisting of a sealed glass tube and the excitation radiation coming from an electrcial discharge supported in an inert gas atmosphere, under a pressure below atmospheric pressure, for example argon, under a pressure of 0.01 to 40 mmHg, contained inside the envelope.
In accordance with the current prior art, the luminescent materials normally used in lamps of the abovementioned kind require so-called xe2x80x9chardxe2x80x9d ultraviolet radiation to excite them, that is to say radiation having a wavelength less than a value of about 300 nm, such as 254 nm radiation produced by the excitation of a small amount of mercury contained in the said inert gas atmosphere.
Such ultraviolet radiation is strongly actinic and the irradiation by this radiation is capable of causing dangerous physiological effects in living creatures. In the manufacture of discharge lamps intended for illumination, although glass envelopes having a high coefficient of ultraviolet radiation absorption are used, it has been found that the emission of xe2x80x9chardxe2x80x9d ultraviolet radiation in the usual fluorescent tubes is not insignificant, so that prolonged exposure to the light emitted by these tubes is not without health risks.
Moreover, the use of mercury, even in small amounts (for example corresponding to a partial pressure of about 0.007 mmHG at 20xc2x0 C.) in the inert gas atmosphere of discharge lamps carries a significant risk of pollution, both during manufacture of the lamps and during disposal of spent lamps, despite the protective measures usually taken.
The invention aims to eliminate the abovementioned risks.
For this purpose, the lamp according to the invention is characterized in that the said luminescent material consists at least partly of a luminescent compound of formula:
Ba7-x-yMxEuyF12XuYv 
in which
M represents at least one divalent metal chosen from celcium, magnesium, strontium and zinc;
x is a number between 0 and 2;
y is a number between 0.00001 and 2;
X and Y each represent an element, which is identical or different, chosen from chlorine and bromine; and
u and v are numbers 0 and 2, the sum of which is equal to 2.
Advantageously, the luminescent material may contain, in addition to the said luminescent compound, auxiliary substances, especially one or more alkali or alkaline-earth metal halides and/or at least one mixed salt of such halides, making it possible to modify its physical properties such as its rheological characteristics and its adhesion to the substrate to which it has to be applied.
Such a luminescent material exhibits high values of light emission intensity, in the visible part of the spectrum, for excitation wavelengths corresponding to the range of so-called xe2x80x9csoftxe2x80x9d ultraviolet radiation, that is to say greater than 300 nanometres, whereas the luminescent materials currently used in the fluorescent tubes have a very low emission intensity for such excitation radiation wavelengths.
This makes it possible, in particular, to use, as excitation radiation, radiation having a 337 nm wavelength generated by an electrical discharge in an atmosphere of molecular nitrogen (N2) [optionally containing a small amount of a rare gas such as neon] under a pressure of about 0.2 to 20 mmHg. The two risks mentioned above are thus eliminated since the discharge lamp is completely free of mercury and it uses excitation radiation much less actinic than that for the fluorescent tubes according to the prior art.
The invention also relates to a luminescent material capable of emitting visible light due to the effect of being irradiated by ultraviolet excitation radiation, characterized in that it consists at least partly of a luminescent compound of formula:
Ba7-x-yMxEuyF12XuYv 
in which
M represents at least one divalent metal chosen from calcium, magnesium, strontium and zinc;
x is a number between 0 and 2;
y is a number between 0.00001 and 2;
X and Y each represent an element, which is identical or different, chosen from chlorine and bromine; and
u and v are numbers between 0 and 2, the sum of which is equal to 2.
The invention furthermore relates to the luminescent compound as defined above.
Advantageously, the said luminescent compound contains, in addition to europium, oxygen in the form of ionized atoms inserted into cationic and/or interstitial sites and into anionic and/or interstitial sites in its crystal lattice respectively, in an amount suitable for activating its light-emitting properties.
According to one particularly advantageous embodiment, the luminescent compound corresponds to the formula:
Ba7-x-yMxEuyF12Cl2 
in which M, x and y have the meanings indicated above.